biology question #3932



Julie, a 11 year old female from Portland, Oregon asks on July 15, 2007,

Q:

How are sand dollars bleached white? Is it by the sun or the sea?

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the answer

Peter Fankboner answered on July 18, 2007, A:

No bleaching is required because the underlying endoskeleton of the sand dollar is naturally white. After they die, their sand dollar outer skin decomposes leaving the white shell exposed.

Sand dollars along with sea stars, serpent stars, sea cucumbers, sea urchins, sea daisies, and sea lilies, make up the family or Phylum Echinodermata which means "spiny-skinned". Echinoderms are exclusively marine, possess an internal skeleton of calcium carbonate ossicles, and are more or less radially pentamerous in their organization (having five sides or legs). Sand dollars (see Figure)  live in organically rich sand, and are covered by a fur of minute, club-shaped, articulated spines (Figure insert) which aid them in burrowing, cleansing and feeding. The sand dollar’s white, calcium carbonate endoskeleton and spines are produced by a thin outer epithelium, which is often highly pigmented. When a sand dollar dies, this epithelium decomposes, leaving behind thousands of loose microscopic spines and an intact chalky white discus-shaped test (shell). While this involves no bleaching process by the sun or the sea per se, breakdown of the sand dollar's epithelium is facilitated by the feeding activities of microorganisms, minute crustaceans and worms.

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